In a general aspect, the present invention relates to electric cables comprising at least one conducting core and at least one insulation coating comprising a polyolefin polymeric base.
More particularly, the present invention relates to an electric cable comprising an insulation coating having an improved resistance against the formation of the so-called water trees.
The present invention also relates to a polymeric composition which has a preferred, although not exclusive, use in the manufacture of an insulation coating in electric cables for power transmission at medium or high voltage.
In the following description, the terms: medium and high voltage, are used to indicate voltages of from 1 to 35 kV and, respectively, over 35 kV.
As is known, one of the more difficult problems to solve in the manufacture of electric cables for power transmission or energy cable, is that of ensuring that the insulation coating of the conducting core achieves an effective control of the electric field generated during the energy transmission, dissipates as little power as possible and preserves its dielectric and structural characteristics along time.
It is also known that to obtain such characteristics the best material for manufacturing the insulation coating is constituted by olefin polymers and in particular by polyethylene and copolymers or terpolymers thereof.
More particularly, among the latter, cross-linked polyethylenexe2x80x94commonly indicated by the acronym XLPExe2x80x94has either good dieletric characteristics or a low loss factor (minimum dissipated power).
But together with these good characteristics, olefin polymers generally possess a low resistance against a particular degradation phenomenon, known in the art by the term: xe2x80x9cwater treeingxe2x80x9d, which may cause in time a degradation of the electric characteristic of the insulation material.
Such phenomenon essentially consists in the formation of microfractures having a branched shape (trees), progressively growing in time and responsible, in some cases, for an electric weakening of the insulation coating.
Even though the mechanism leading to the formation of these microfractures or weak zones has not yet been fully clarified, the formation of such zones or xe2x80x9ctreesxe2x80x9d is anyhow attributed to the combined action of the electric field generated by the current flow in the conducting core of the cable and of the moisture existing in the inside of the insulation coating.
The problem represented by the formation of the above water trees is particularly felt in the cables for power transmission at medium or high voltage which are not provided with external protection elements, wherein the insulation coating may be directly in touch with water or anyhow with humid environments.
In order to reduce somehow the formation of water trees, different solutions have been proposed in the art, which are essentially based either on the selection of suitable polymeric materials for the manufacture of the insulation coatings, or on the use of suitable retarding additives, so-called tree-retardants.
So, for instance, it is known from U.S. Pat. No. 5,246,783 the use of an ethylene copolymer and of an alpha-olefin having 3 to 20 carbon atoms, having a molecular weight distribution of from 1.5 to 30 and a distribution index of the alpha-olefin comonomer greater than 45%.
European Patent EP 0 179 845 discloses, on the other hand, the combined use of an ethylene polymer or of an ethylene copolymer with an alpha-olefin, with an ethylene-alkylacrylate or an ethylene-alkylmethacrylate copolymer, in a cross-linkable coating composition resistant to the formation of water trees, for medium/high voltage energy cables.
As to the use of the so-called tree-retardant additives, it is known from the U.S. Pat. Nos. 4,212,756 and 4,144,202 the use of particular organo silanes comprising an acrylic/methacrylic group and, respectively, an epoxy group.
According to the present invention, it has now been found that the simultaneous presence of ester groups and epoxy groups in a polyolefin polymeric composition may impart to said composition a particular resistance to the phenomenon of water trees in the working condition of an electric cable.
According to the invention, in fact, a surprising synergistic effectxe2x80x94in terms of increased resistance to the water treeing phenomenonxe2x80x94has been observed, due to the simultaneous presence of ester groups and epoxy groups within a selected concentration range.
When the aforementioned groups are simultaneously present in the polymeric base forming the insulation coating of the cable, it has been noticed in particular that the retarding effect against water tree formation is evidently greater than the sum of the effects of the same groups when the latter are present alone.
According to a first aspect thereof, the present invention therefore provides an electric cable comprising at least a conductor and at least an insulation coating comprising a polyolefin polymeric base, which is characterized in that said polyolefin polymeric base comprises, in parts by weight to the total weight of the same:
from 0.5 to 15 parts of ester groups; and
from 0.01 to 5 parts of epoxy groups.
According to a further aspect of the invention, the problem of an adequate resistance to the phenomenon of water trees is therefore solved by a cable which is characterized in that said polyolefin polymeric base comprises a first predetermined amount of ester groups and a second predetermined amount of epoxy groups, said first and second predetermined amounts being such as to reduce the water tree formation in the insulation coating material after electric ageing in water.
In the following description and the subsequent claims, the term: electric water ageing, is used to indicate an ageing treatment of the insulation coating carried out in water and in the presence of an electric field such asxe2x80x94for instancexe2x80x94the treatment proposed by EFI (Norwegian Electric Power Research Institute), illustrated below, or analogous treatments well known in the art.
According to a further aspect thereof, the present invention also provides a polyolefin polymeric composition resistant to water treeing, in particular for the manufacture of an insulation coating for electric cables, which is characterized in that it comprisesxe2x80x94in parts by weight to the total weight thereofxe2x80x94from 0.05 to 15 parts of ester groups and from 0.01 to 5 parts of epoxy groups.
In the following description and the subsequent claims, the term: polyolefin polymeric base, is used to indicate a polymer selected from the group comprising high-, medium- and low-density polyethylene homopolymers, ethylene copolymers and ethylene terpolymers with an alpha-olefin having 3 to 20 carbon atoms, ethylene-alpha-olefin-diene terpolymers and mixtures thereof.
The term: polyolefin polymeric composition, on the other hand, is used to indicate a polymeric composition comprising a polyolefin polymeric base of the above defined type.
Preferably, the polyolefin polymeric base of the invention is an ethylene polymer selected from the group comprising: polyethylene, copolymers obtainable by polymerizing ethylene with at least one alpha-olefin, linear or branched, having 3 to 14 carbon atoms, terpolymers obtainable by polymerizing ethylene, an alpha-olefin, linear or branched, having 3 to 14 carbon atoms and a diene having 4 to 25 carbon atoms having a density (measured according to ASTM D-792) of from 0.860 g/cm3 to 0.940 g/cm3 and a Melt Index (measured according to ASTM D-1238) of from 0.1 g/10xe2x80x2 to 40 g/10xe2x80x2.
In the terpolymers of the invention, the above diene is preferably selected from the group comprising: 1,4 pentadiene, 1,4 hexadiene, 1,5 hexadiene, dicyclopentadiene, 4-vinyl-cyclohexene, 1-vinyl-1-cyclopentene, ethyl norbornene (LNB), alkylbicyclononadiene, indene, norbornene and mixtures thereof.
According to the invention, it has been observed that to achieve an adequate resistance to the water tree formation, the polyolefin matrix forming the insulation coating of the cable conducting core should preferably comprise at least 0.5% by weight of ester groups and at least 0.01% by weight of epoxy groups.
On the other hand, it has been observed that amounts exceding 15% by weight of ester groups and, respectively, 5% by weight of epoxy groups do not produce a substantial additional benefit in terms of resistance to the phenomenon of water trees, against a marked increase in the power dissipated by the insulation coating (increase of the loss factor or tg delta), with ensuing increase in the energy transmission costs.
According to the invention, is has also been observed that the aforementioned improved resistance to the water trees formation is not substantially affected by the way in which the ester groups and the epoxy groups are incorporated into the polymeric composition, provided that these groups are present in the aforementioned amounts, as indicated above.
So, for instance, in a first embodiment of the invention, a polymeric composition comprising the above minimum amount of ester groups and epoxy groups may be prepared by adding to a polyolefin polymeric base a first compound, either polymeric or not, incorporating an ester group and, respectively, a second compound, either polymeric or not, incorporating an epoxy group.
Preferably, the above compound incorporating an ester group is an acrylic or vinyl polymer selected from the group comprising:
i) copolymers obtainable by polymerizing ethylene with at least an acrylic ester of the formula: 
xe2x80x83wherein R1 is H or CH3, R2 is an alkyl or aryl hydrocarbon group, linear or branched, preferably a phenyl, having 1 to 10 carbon atoms, and R3 is hydrogen or an alkyl or aryl hydrocarbon group, preferably a phenyl, linear or branched, having 1 to 10 carbon atoms;
ii) copolymers obtainable by polymerizing ethylene with at least a vinyl ester of a carboxylic acid of the formula: 
xe2x80x83wherein R2 is an alkyl or aryl hydrocarbon group, linear or branched, preferably a phenyl, having 1 to 10 carbon atoms.
For the purposes of the invention, the acrylic copolymers having the formula (I) of preferred use comprise ethylene copolymers with a comonomer selected from the group comprising the following acrylic esters: methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, iso-butyl acrylate, methyl methacrylate, ethyl methacrylate, and mixtures thereof.
Among them, particularly preferred are the acrylic ester-ethylene copolymers and, more particularly, those having an amount of acrylic comonomer of from 2% to 40% by weight, such as for instance those commercially available under the trade names ENATHENE EA(trademark) (Quantum Chemical Corporation, Cincinnati, Ohio, U.S.A.), OPTEMA(trademark) (Exxon Chemical) and LOTRYL(trademark) (Elf Atochem).
For the purposes of the invention, the vinyl copolymers of preferred use comprise ethylene copolymers with a comonomer selected from the group comprising vinyl acetate and vinyl propionate, such as for instance those commercially available under the trade names LEVAPREN(trademark) (Bayer), ESCORENE(trademark) (Exxon Chemical), ELVAX(trademark) (Du Pont de Nemours International S.A.) and EVATANE(trademark) (Elf Atochem).
According to an advantegeous aspect of the invention, optimal homogeneity characteristics of the polymeric composition may be achieved when the acrylic or vinyl copolymers having the formula (I) and (II) have a Melt Index value near that of the polyolefin polymeric base in which they may be easily incorporated.
For the purposes of the invention, such Melt Index value (measured according to ASTM D-1238) preferably ranges from 0.1 g/10xe2x80x2 to 40 g/10xe2x80x2.
According to a preferred embodiment, the above amount of ester groups may be reached when the polymeric composition comprises at least 5% by weight of the above acrylic or vinyl polymers having the formula (I) and (II), or greater values depending upon the ester groups content within said polymers.
Furthermore, the polymeric composition of the invention preferably comprises from S% to 40k by weight of the aforementioned acrylic or vinyl polmers having the formula (I) and (II).
According to the invention, the above-identified compound incorporating an epoxy group may be:
a) a glycidyl ether of the formula: 
xe2x80x83wherein R4, R6 and R7 are independently a hydrogen atom, an alkyl or aryl hydrocarbon group, linear or branched, preferably an optionally substituted phenyl, having 1 to 25 carbon atoms or a glycidyl ether group of the formula: 
b) a polyfunctional derivative of the glycidyl ether of formula (III), or
c) an epoxy resin.
For the purposes of the invention, the glycidyl ethers having the formula (III) may be mono-, bi-, tri- or tetrafunctional; those of preferred use are mono- and bifunctional glycidyl ethers selected from the group comprising: p-t-butyl-phenyl-glycidyl ether, 2-ethyl-hexyl-glycidyl ether, dodecyl-glycidyl ether, tetradecyl-glycidyl ether, glycidyl-isopropyl-etherbutyl-glycidyl ether, 1,4-butanediol-diglycidyl ether, 1,6-hexen-diol-diglycidyl ether and mixtures thereof.
Epoxy resins of preferred use comprise instead resins of the aliphatic, cycloaliphatic or aromatic type, preferably having a dynamic viscosity at 25xc2x0 C. of from 60 to 55,000 mPaxc3x97s, preferably between 7,000 and 10,000 mPaxc3x97s, and an epoxy content (measured according to ASTM D-1652) of from 0.1 to 0.7 gram eq/100 g of resin, preferably between 0.53 and 0.55 gram eq/100 g of resin.
Examples of such resins are those commercially available under the trade name EUREPOX(trademark) (SCHERING) and preferably those named EUREPOX(trademark) 730.
Preferably, the polymeric composition comprises from 0.2 to 10% by weight of at least one glicydyl ether having the formula (III) or polyfunctional derivatives thereof and/or from 0.2 to 10% by weight of said epoxy resin, so as to reach the aforementioned amount of epoxy groups indicated hereinabove.
According to a further embodiment of the invention, the above-identified minimum amount of ester groups and epoxy groups may be reached by adding to a polyolefin polymeric base a compound, either polymeric or not, incorporating either an ester group or an epoxy group.
Bifunctional compounds of preferred and advantageous use are, more particularly, those selected from the group comprising the glycidyl esters of the formula: 
wherein R3 and R5 are independently H, an alkyl or aryl hydrocarbon group, linear or branched, preferably a phenyl, having 1 to 10 carbon atoms; R1 is H or CH3.
Among them, the glycidyl esters of the acrylic or methacrylic acid are preferred.
For the purposes of the invention, particularly preferred is glycidyl methacrylate (GMA), commercially available under the trade name BLEMMER G(trademark) (Blemmer Chemical Corp.).
In this case, the aforementioned amount of ester and epoxy groups may be reached when the polymeric composition preferably comprises from 0.03 to 15% by weight of at least a glycidyl ester having the formula (IV).
Bifunctional compounds of the polymeric type of preferred and advantageous use are those selected from the group comprising:
a) terpolymers obtainable by polymerizing ethylene with:
i) at least one acrylic ester of the formula: 
xe2x80x83wherein R1 is H or CH3, R2 is an alkyl or aryl hydrocarbon group, linear or branched, preferably a phenyl, having 1 to 10 carbon atoms and R3 is hydrogen or an alkyl or aryl hydrocarbon group, preferably a phenyl, linear or branched, having 1 to 10 carbon atoms;
ii) at least one glycidyl ester of the formula: 
xe2x80x83wherein the meaning of R1, R3 and R5 are those indicated hereinabove;
b) terpolymers obtainable by polymerizing ethylene with:
i) at least one vinyl ester of a carboxylic acid of the formula: 
xe2x80x83wherein R2 is an alkyl or aryl hydrocarbon group, linear or branched, preferably a phenyl, having 1 to 10 carbon atoms;
ii) at least one glycidyl ester of the formula: 
xe2x80x83wherein the meaning of R1, R3 and R5 are those indicated hereinabove;
c) copolymers obtainable by polymerizing ethylene with at least one glycidyl ester of the formula: 
xe2x80x83wherein R3 and R5 are independently H, an alkyl or aryl hydrocarbon group, linear or branched, preferably a phenyl, having 1 to 10 carbon atoms; R1 is H or CH3.
Also in these cases, glycidyl esters having the formula (IV) of preferred use are the glycidyl esters of the acrylic or methacrylic acids and, particularly, glycidyl methacrylate.
Ethylene/acrylic ester/glycidyl methacrylate terpolymers and ethylene/glycidyl methacrylate copolymers of preferred and advantageous use are commercially available under the trade names LOTADER(trademark) GMA AX8900 and LOTADER(trademark) GMA AX8840 (Elf Atochem), respectively.
As previously explained, in this case too optimum homogeneity characteristics of the polymeric composition may be obtained when the above bifunctional terpolymers or copolymers have a Melt Index of from 0.1 g/10xe2x80x2 to 40 g/10xe2x80x2.
In this case, the aforementioned amount of ester and epoxy groups may be reached when the polymeric composition comprises from 3 to 30% by weight of at least an ethylene/acrylic ester/glycidyl methacrylate terpolymer or ethylene/vinyl ester/glycidyl methacrylate terpolymer and from 1 to 40 by weight of at least an ethylene/glycidyl methacrylate copolymer.
Obviously, both the bifunctional compounds, either polymeric or not, may be used in the polymeric composition of the invention, either alone or combined with the aforementioned monofunctional compounds (acrylic or vinyl polymers, glycidyl ether, epoxy resin), so as to reach the desired amount of ester and epoxy groups.
According to a further aspect thereof, the present invention relates to a new use of one of the above bifunctional compoundsxe2x80x94either polymeric or notxe2x80x94incorporating either an ester group or an epoxy group, as a tree retardant additive in a polymeric composition for coating an electric cable.
In fact, it is advantageously possible to confer to a polyolefin polymeric base the desired characteristics of resistance to the water treeing phenomenon by simply adding said bifunctional compounds to said base in the aforementioned amounts.
Preferably, the amount of the ester and epoxy groups falls within said range of 0.5-15% parts by weight and, respectively, of 0.01-5% parts by weight to the total weight of the composition so obtained.
In a preferred embodiment, the polymeric composition of the invention is cross-linked by means of one of the methods known in the art to this end.
Preferably, the polymeric composition is chemically cross-linked; for this purpose, it incorporates an effective amount of at least one cross-linking agent, such as for instance tei-butyl-cumyl peroxide.
In order to achieve an improved stability, furthermore, the polymeric composition of the invention advantageously incorporates an effective amount of at least one antioxidant agent, such as for instance 4,4xe2x80x2-thio-bis(3-methyl-6-ter-butyl)phenol.
Depending upon the particular use of the cable, moreover, the polymeric composition of the invention may incorporate other additives and fillers conventional in themselves; such as instance pigments, dyes, stabilizers, lubricants, etc.